Disc drives are the primary devices employed for mass storage of computer programs and data used in computer systems. Within a disc drive, a load beam supports a hydrodynamic air bearing (or slider) proximate a rotating magnetic disc. The load beam supplies a downward force that counteracts the hydrodynamic lifting force developed by the air bearing. The slider carries a magnetic transducer for communicating with individual bit positions on the rotating magnetic disc.
Upon disc drive power-up, it is necessary for the disc to accelerate from an at rest, non-rotating condition, to an operational condition in which the disc rotates with sufficient speed that the slider is able to fly over the disc at an operational height.
In order to initiate rotation, various resistive forces must be overcome. First, during rest, every head in the drive generally sticks to the disc surface to some degree, this is known as stiction. The stiction due to the magnetic heads increases proportionally with the number of magnetic heads employed in the drive. Second, some disc drives employ additional air bearings between the rotational and non-rotational members of the drive. For such drives, additional stiction is also present between such air bearing surfaces. Thus, the excess of spindle motor torque over the resistive forces such as stiction, goes into accelerating the rotational mass.
In the field of disc drives, it is advantageous to provide a disc drive which can initiate disc rotation and achieve operational disc speed as quickly as possible. In disc drive spindle motors, generally, spindle torque is proportional to the amount of current which is caused to flow through the coils of the spindle motor. Thus, accurate and precise control of coil current is advantageous in repeatedly providing the maximum safe energization current to the spindle motor to generate the maximum safe torque.
The present invention addresses these and other problems, and offers other advantages over the prior art.